Semiconductor device geometries (i.e., integrated circuit design rules) have decreased dramatically in size since integrated circuit (IC) devices were first introduced several decades ago. ICs have generally followed “Moore's Law,” which means that the number of devices fabricated on a single integrated circuit chip doubles every two years. Today's IC fabrication facilities are routinely producing 65 nm (0.065 μm) feature size devices, and future fabs will soon be producing devices having even smaller feature sizes.
In most IC fabrication facilities, part of the fabrication process involves employing plasma in process equipment to either react or facilitate a reaction with a substrate such as a semiconductor wafer. Radio frequency (RF) or microwave power supply generators are widely used in semiconductor and industrial plasma processing equipment to generate the plasmas in a process chamber. Plasma processing is used for a wide variety of applications including etching of materials from substrates, deposition of materials onto substrates, cleaning of substrate surfaces, and modification of substrate surfaces.
With reference to FIG. 1, a cross-sectional view of a portion of a prior art plasma-based process system 100 includes a vacuum chamber 109 having a chamber wall 101. At least a portion of the chamber wall 101 includes a window 103, typically formed of quartz or a similar material which is transparent to radio frequency (RF) waves at the operating frequencies of the plasma-based process system 100. A coil 115 is located outside of the vacuum chamber 109 and surrounds the window 103. An RF generator 119 is connected across the coil 115 through a matching network 117. The matching network 117 provides a means of matching an output impedance of the RF generator 119 to an input impedance of the vacuum chamber 109. The matching network 117 may contain fixed elements only, or it may contain elements such as variable capacitors and variable inductors allowing dynamic impedance matching of the RF generator 119 to changing load conditions.
The RF generator 119 in the plasma-based process system 100 is the primary plasma generating portion provided to energize the plasma in the vacuum chamber 109. The plasma could also be generated by a capacitive electrode (not shown) immersed in the plasma or by an inductive coil (not shown) immersed in the plasma.
Process gases are introduced into the vacuum chamber 109 from a plurality of feed gas supply reservoirs 105 (only one is shown). A vacuum pump 107 is configured to establish a vacuum level within the vacuum chamber 109 suitable for a particular process (e.g., plasma etch or deposition) being performed in the plasma-based process system 100. The vacuum pump 107 is further configured to exhaust process gases from the vacuum chamber 109.
A substrate 113 (e.g., a semiconductor wafer) is supported in the vacuum chamber 109 by a substrate support, typically an electrostatic chuck (ESC) 111. The ESC 111 acts as a cathode within the plasma-based process system 100 and develops an electrostatic potential to hold the substrate 113 in place during processing, thus avoiding the problems of mechanical clamping by having contact with only the back side of the substrate 113. The ESC 111 operates by inducing opposing charges between the substrate 113 and the ESC 111 thereby resulting in an electrostatic attraction between the two pieces. Thus, a resulting capacitance exists between the ESC 111 and the substrate 113. The capacitance is large enough such that an insignificant drop in RF voltage exists between the ESC 111 and the substrate 113 during processes performed at the high frequencies involved.
The plasma-based process system 100 further includes an anode (not shown directly although the anode is typically formed of the chamber wall 101 and/or chamber top) within the vacuum chamber 109. To process the substrate 113, a reactive gas is pumped from one or more of the plurality of feed gas supply reservoirs 105 into the vacuum chamber 109. The anode and ESC 111 (acting as a cathode) are driven by a single sinusoidal frequency from the RF generator 119 to excite the reactive gas into a plasma. The single frequency is typically 13.56 MHz, although single frequencies from 100 kHz to 2.45 GHz are often used, with an occasional use of other single frequencies. More specifically, a single frequency, sinusoidal RF signal is generally applied to the reactive gas within the chamber at a relatively high-power level, for example, at 3 kilowatts. The RF power excites the reactive gas, producing the plasma within the vacuum chamber 109 proximate to the substrate 113 being processed. The plasma-enhanced reactive ion process is commonly employed in etch and chemical vapor deposition processes.
More recently, multiple sinusoidal frequencies have been used to excite the plasma within a vacuum chamber. In these systems, a cathode/anode bias circuit is driven with a first RF frequency and an antenna or coil, proximate to the vacuum chamber, is driven with a second RF frequency. Thus, each circuit is coupled to a separate and distinct RF power delivery system consisting of separate RF oscillators, preamplifiers, power amplifiers, transmission lines, and matching networks that supply independent sinusoidal RF frequencies at high power levels to each of the plasma excitation circuits. The redundant oscillators and other associated circuitry are costly and complex. In other systems, both the first and second frequencies are supplied to a single electrode but frequently suffer from a reduction in power transfer since both frequencies cannot be matched concurrently.
If impedances of the power source and the load (i.e., the element coupled to the plasma) are not matched, the power supplied to (or absorbed by) the load is reflected and power transfer is therefore not maximized. Thus, controlling the amount of power absorbed or reflected by the load is important. Additionally, unmatched impedances can be detrimental to the power source or to other components coupled to the power source. In most cases the load impedance (i.e., the input impedance of the plasma coupling element) cannot be determined in advance, since it is dependent on the state or condition of the plasma to which it is coupled, and the plasma state can vary during processing. Accordingly, many plasma processing systems utilize a matching network provided between the RF source and the plasma coupling element to match input and output impedances. The matching network is utilized in order to maximize the amount of RF power supplied to the plasma, and to control the amplitude and phase of the bias power.
Recently, studies indicate that tailoring of ion energy in a plasma reactor can be used advantageously in semiconductor wafer processing. Complex radio frequency (RF) waveforms can be generated and delivered to a bias electrode of the plasma reactor and used to produce desirable effects in the generated plasma. The complex RF waveforms may contain ten or more harmonic frequencies or as few as two (a fundamental frequency with a second phase-shifted harmonic).
However, the complex waveforms are difficult to match to the plasma load due to the presence of the harmonic frequencies. Prior art matching systems described above are only capable of matching a single frequency RF waveform to a plasma chamber and are therefore incapable of providing adequate bandwidth for a complex waveform comprised of various frequencies. Moreover, an impedance of the apparent plasma load can be dependent on the frequency of the input waveform. Therefore, a system for matching the complex waveforms to the plasma reactor is required to efficiently match the impedance of each of the harmonic frequencies to the plasma generator.